In construction machines such as hydraulic excavators and hydraulic cranes, the components thereof are driven by a hydraulic drive system. Conventionally, in such a hydraulic drive system, a hydraulic static transmission (HST) is used in a running circuit. In recent years, a hydraulic drive system in which HST is used in a turning circuit has been proposed. For example, Patent Literature 1 discloses a hydraulic drive system 100 as shown in FIG. 9.
In the hydraulic drive system 100, a variable displacement bi-directional pump (also called an over-center pump) 110 is connected to a turning motor 120 by a pair of supply/discharge lines 131 and 132, such that a closed loop is formed. The supply/discharge lines 131 and 132 are connected to each other by a bridging passage 133. The bridging passage 133 is provided with a pair of relief valves 141 and 142, which are directed opposite to each other.
A tank line 134 extends from a part of the bridging passage 133 to a tank, the part being positioned between the relief valve 141 and the relief valve 142. A charge pump 160 is connected to the tank line 134. Hydraulic oil discharged from the charge pump 160 and/or hydraulic oil that has passed through the relief valve (141 or 142) is/are supplied, for replenishment, to one of the supply/discharge lines 131 and 132 via a check valve (151 or 152).
The hydraulic drive system 100 is configured such that a swash plate 111 of the pump 110 is directly moved by pilot pressures a1 and b1 outputted from a turning operation valve 170.